1. Field of the Invention
The present invention is directed to a deactivatable anti-theft security strip of the type used in security systems making use of an alternating filed to detect the unauthorized removal of articles to which the security strip is attached.
2. Description of the Prior Art
Security systems are known which make use of an alternating field in an examination zone, such as at an exit of a store, with all articles in the secured area having an anti-theft security strip attached thereto, the strip becoming magnetically reversed in the alternating field in the examination zone and consequently causing the generation of pulses having characteristic harmonics in a reception coil. Such strips are known which consist of an elongated composite member having a soft-magnetic constituent and a hard-magnetic constituent, with the two constituents forming the composite member being in mechanical contact so that each constituent mechanically supports the other.
Such an anti-theft security strip is disclosed in German OS No. 29 33 337. This strip is formed by a ferromagnetic wire having two layers supported relative to each other. The magnetically hard, interior layer consists, for example, of a Fe-Co-V alloy which is exteriorly surrounded by a second constituent consisting of soft steel, Cu, Ni, Al or brass.
A number of manufacturing techniques for such composite wires or composite strips are disclosed, for example, in German Patent No. 31 52 008. The use of an elongated magnetic switch core is disclosed therein, the core having a circular or angular cross section and typically being magnetically reversed in the presence of a discontinuous field strength of approximately 15 A/cm.
German OS NO. 38 24 075 discloses a composite element which is specifically optimized for use in anti-theft security systems. The composite element is referred to as a pulse wire, and is formed by a core and sheath of two respectively different magnetic materials. The hard-magnetic sheath is magnetized in the activated condition. When the pulse wire proceeds into the alternating field of an examination zone, the soft-magnetic core is suddenly magnetically reversed at every other half-wave of the alternating field.
The magnetized, hard-magnetic sheath diminishes the demagnetization tendencies at the end of the pulse wire so that relatively short wires can be used, however, this means that the amplitude of the alternating field must be high enough to overcome the pre-magnetization of the hard-magnetic sheath. In this known pulse wire, the response field strength also therefore lies at approximately 15 A/cm.
An elongated pulse wire having a low response field strength of approximately 0.8 A/cm is disclosed in U.S. Pat. No. 4,660,025. The pulse wire disclosed therein is a wire consisting of amorphous metal which, due to the nature of its manufacture, has internal stresses and therefore experiences a fast magnetic reversal, the fast magnetic reversal being due at least in part to the internal stresses.